This invention relates to metallurgical melting processes and more particularly to a method of and apparatus for accurately regulating the gaseous fuel-air mixture employed for providing heat and for providing a proper atmosphere for a copper melting furnace.
Many metallurgical processes are carried out in an environment characterized by high temperatures and a gaseous atmosphere of closely controlled chemical composition. An example of one such process is that carried out in a copper melting furnace. Other examples are heat treating furnaces or surface treatment furnaces such as carburizing ovens in which close atmosphere control is required. To achieve this controlled atmosphere, many prior art processes use electric heaters to provide primary heating and a separate combustion gaseous fuel supply to provide an atmosphere of controlled chemical composition. A more advantageous way to carry out these processes is to provide a single source of hydrocarbon fuel, e.g., natural gas or methane, propane, butane or the like which, upon combustion with air or oxygen, supplies both the required heat and an atmosphere having the required chemical composition. However, many problems arise when adapting a combustion system to fulfill two diverse objectives. One major problem is accurate control of the chemical composition of the mixture which is to be combusted. This is most important because this initial composition has the greatest effect on the combustion process and once this initial composition is known or fixed, the resulting products of the combustion process can be accurately predicted.
It is especially troublesome to maintain accurate control of the fuel and air mixing over a wide range of varying flow rates which are often required due to the varying heat demands of some metallurgical processes, e.g., the melting of copper to supply a variable rate casting process.
The ratio of air to fuel influences both the combustion temperature and the composition of the products of combustion. If the mixture contains excess air, the flame is relatively cool and the products of combustion contain unreacted oxygen. If the mixture contains excess fuel the flame is much hotter and the products of combustion contain unreacted hydrogen.
More precisely, it is the mass ratio of fuel to available oxygen in the air which most influences the combustion process. However there are other variables, such as the temperature and humidity and density of the air which also have a secondary influence on the combustion process as explained in detail hereinafter.
For example, the temperature of the ambient air used as a source of oxygen may vary as much as 40.degree. F. during any given day, causing about a 3.5% change in the mass flow of oxygen at a constant volume flow of air thus changing the composition of the combustion gases. Such a variation has a significant effect on the atmosphere generated by combustion in a metallurgical melting furnace and can adversely affect the quality of the product produced. Similarly, variations in the humidity of the ambient air can have a significant effect on the oxygen content of a given volume of air, particularly at high temperatures. Thus, for example, at 110.degree. F., a variation in humidity from 0% to 100% of the ambient air causes a reduction of the oxygen content of the air of about 7%. This reduction of oxygen content can have a significant and deleterious effect on many metallurgical processes. However, more importantly, changes in humidity of the incoming air stream have a pronounced effect on the chemical composition of the combustion gases due to the equilibrium reactions of the combustion process. For example, a high level of water vapor in the reactant stream causes an increase in the water vapor level of the product stream which inhibits complete combustion due to the well known chemical rules.
Conventional systems generally mix fuel and air on a volume flow basis and therefore do not supply a constant stoichiometric mass ratio of fuel to oxygen under varying operating conditions. One such mixing device is disclosed in U.S. Pat. No. 3,799,195.
Other U.S. Pat. Nos. which have been uncovered relating to combustion gas mixing devices are as follows: 3,883,322, 3,934,987, 3,788,825, 3,230,059, 3,721,253. These patents are generally concerned with vaporizing and/or mixing one or more gaseous hydrocarbons for combustion but are not addressed to the problems associated with regulating the mass flow of combustible gases to provide both heat and a precise non-oxidizing atmosphere in a metallurgical process, e.g., copper melting furnace.
In melting copper, care must be taken to limit the oxygen content added to the melted copper to as low a value as possible, preferably zero. In practice, however, this goal is difficult to attain due to the normal variations of the furnace atmosphere generated during the combustion process. Although it is generally desirable to limit the oxygen content to below about 0.045% (by weight), at varying times this limit can be exceeded. If the oxygen content of the metal exceeds about 0.05% the copper is brittle and must be remelted and/or deoxidized to reduce the oxygen content. In practice an oxygen content of about 0.03% or less is preferred.
In U.S. Pat. No. 3,199,977 there is disclosed one type of copper melting furnace which employs a combustion system designed to operate on natural gas so as to yield cast copper bar with an oxygen content of from 0.01 to 0.035%. The patentees recognize that inadequate fuel-air mixing can result in cast bars with greater than desired oxygen levels. However, in this patent the desired mixing control is accomplished by providing an off-centered orifice plate upstream of a mixing elbow which directs air into the fuel stream in a predetermined way which is empirically derived. This patent does not recognize, nor does it deal with, variations in fuel-air mass flow ratios resulting from pressure-temperature fluctuations in one or both of the gaseous components used in the combustion process and thus the apparatus described in the patent must be constantly monitored and adjusted. In addition, since an adequate supply of natural gas is not always available, it would be desirable to operate a combustion system on liquified gas. However difficult problems arise when attempting to accurately control combustion by mixing air with fuel on a volume basis. It is known that variations in the temperature or pressure of a gas will change the mass per unit volume but it is not practical to maintain all the variables at a constant value due to the very large volumes and high flow rates involved.